(1) Field of the Invention
The present invention relates to a developing device as well as to an image forming apparatus and toner supplying method using the device, in particular relating to a developing device using a dual-component developer containing a toner and a magnetic carrier, for use in an image forming apparatus for forming image using the toner based on electrophotography, such as an electrostatic copier, laser printer, facsimile machine or the like, as well as to an image forming apparatus and toner supplying method using this device.
(2) Description of the Prior Art
Conventionally, image forming apparatuses based on electrophotography such as copiers, printers, facsimile machines and the like have been known. The image forming apparatus using electrophotography is constructed so as to form an image by forming an electrostatic latent image on the photoreceptor drum (toner image bearer) surface, supplying toner to the photoreceptor drum from a developing device to develop the electrostatic latent image, transferring the toner image formed on photoreceptor drum by development to a sheet of paper or the like, and fusing the toner image onto the sheet by means of a fusing device.
Recently, in the image forming apparatuses capable of reproducing full-color and high-quality images, a dual-component developer (which will be referred to hereinbelow as simply “developer”), which can present excellent charge performance stability, is often used.
This developer consists of a toner and a carrier, which are agitated in the developing device and frictionally rubbed with each other to produce appropriately electrified toner.
In the developing device, the electrified toner is supplied to a developer supporting member, e.g., the surface of a developing roller. The toner thus supplied to the developing roller is moved by electrostatic attraction to the electrostatic latent image formed on the photoreceptor drum. Hereby, a toner image based on the electrostatic latent image is formed on the photoreceptor drum.
Besides, recently there is a demand for high-speed performance and miniaturization on image forming apparatuses, it is hence necessary to electrify the developer quickly and sufficiently and also convey the developer rapidly.
To deal with such circumstances, in order to promptly diffuse supplied toner into the developer and produce an appropriate amount of static charge on the toner, a circulating type developing device has been adopted in some image forming apparatuses. This circulating type developing device includes a developer conveying passage through which the developer is circulatively conveyed and a developer conveying member which agitates and conveys the developer through the developer conveying passage (see patent document 1: Japanese Patent Application Laid-open Hei 10 No. 63081).
The above-mentioned circulating type developing device is constructed such that toner is supplied from a toner hopper to the developer conveying passage when the toner concentration in the developer inside the developing device becomes lower than a predetermined level.
However, since the specific weight of the toner in the developer is rather low compared to that of the carrier (about one third), the supplied toner is likely to float over the developer. As a result, the conventional circulating type developing device has the problem that if a large amount of toner is supplied from the toner hopper to the developer conveying passage, the toner is conveyed as it remains in lumps over the developer.
As a result, there occurred such situations that the toner that has not been sufficiently mixed with the carrier (the toner that has a lower amount of static charge without being sufficiently electrified) is supplied to the developing roller.
That is, if the toner that has not been sufficiently electrified is supplied to the developing roller, the toner becomes prone to scatter from the developing roller, hence giving rise to the problem that the scattered toner soils the interior of the image forming apparatus and the images formed on the paper.
Further, if the toner that has not been sufficiently charged is supplied to the developer supporting member, there occurs the problem that the electrostatic force for retaining toner particles on the carrier surface becomes weak and the toner particles are prone to adhere to non-image areas and produce a foggy image.
In contrast, if the space over the developer in the developer conveying passage is totally eliminated, excess stress acts on the developer, causing the problems that the abrasion of the developer is accelerated so that the life of the developer is shortened, and that the developer becomes unable to be conveyed. The reason of these problems can be considered as follows. That is, the bulk density of the developer is always changing depending of the toner concentration in the developer and the amount of static charge on toner, but if there is no space for absorbing the change, the internal pressure on the developer becomes higher. Particularly, there has been the problem that when the bulk density of the developer becomes higher near the toner supply port, the supplied toner bridges the space around the toner supply port, causing toner blocking.
On the other hand, if an agitating paddle with a high agitating capability is provided, it is possible to improve the performance of agitation and mixture between the supplied toner and the developer, but there is still the problem that excessive stress acts on the developer, hence abrasion of the developer is promoted and the life of the developer is shortened.